1. Field of Invention
This invention relates to active mode sensor systems. Specifically, the present invention relates to coherent active mode sensor systems employing lasers, radars, infrared, microwave, or other type of electromagnetic energy to measure target rotation and range.
2. Description of the Related Art
Active mode sensor systems, such as ladar (laser radar) systems, are employed in various applications including high-resolution 3-dimensional imaging, mapping, chemical analysis, atmospheric analysis, and military targeting applications. Such applications require accurate, space-efficient, and cost-effective ladar systems that can effectively detect and identify a target. The ladar systems must effectively reduce or cancel signal interference, such as platform noise, background clutter, and atmospheric distortion inherent in laser return signals.
An exemplary ladar system includes a sensor suite mounted on a satellite, missile system, aircraft, tank, or other vehicle. The sensor suite has one or more fixed physical apertures through which the ladar system views a scene. The ladar system views the scene by transmitting a laser through the aperture toward the scene. The laser reflects off the scene, producing a laser return that is detected by the ladar system. Many conventional radar and ladar systems measure the intensity of the return beam and the round trip delay from transmission to detection, which yields the distance (range) to the scene. Laser return intensity and range information may be combined with other image information to facilitate target tracking, terrain mapping, atmospheric analysis, and so on.
Ladar systems are either coherent or noncoherent. Coherent ladar systems transmit a laser beam with a predetermined phase and frequency. Knowledge of the spectral characteristics of the transmitted laser beam enables coherent ladar systems to record additional information about the scene, such as target movement and velocity (range rate), and to further improve Signal-to-Noise Ratio (SNR) over corresponding noncoherent ladar systems.
Accurate and efficient radar and ladar systems are particularly important in targeting applications, such as air-to-air combat and missile defense systems, where undesirable noise from atmospheric distortion, platform vibration and background reflections (clutter) is common. Such noise often degrades target detection and tracking accuracy and prohibits effective target vibration sensing.
To reduce undesirable noise, some ladar systems include complex platform stabilization systems to reduce platform vibrations. These platform stabilization systems are often undesirably expensive and bulky and often inapplicable to missile and aircraft applications, where platform noise is difficult or impossible to eliminate. Furthermore, platform stabilization systems typically do not cancel noise due to background clutter or atmospheric distortion of the ladar beam.
To reduce atmospheric distortion and related noise, some ladar systems employ complex electronic hardware and software to perform statistical analysis on the atmosphere to estimate and eliminate the atmospheric noise component in the laser return signal. Unfortunately, statistical atmospheric analysis is often prohibitively expensive and may not sufficiently reduce atmospheric noise for some applications. Furthermore, statistical atmospheric analysis does not typically reduce platform noise.
To reduce both platform noise and noise due to atmospheric distortion and/or background clutter, ladar systems may also employ special noise reduction filters and corresponding systems. These filters may be undesirably expensive and bulky for applications with stringent space constraints, such as missile ladar system applications. Furthermore, such filters often cannot lower noise to acceptable levels required for accurate target tracking.
Alternatively, platform noise, atmospheric noise, and clutter may be reduced by employing independent lasers transmitting at different wavelengths. Noise effects common to both lasers are subtracted from the target return signals as common mode noise. Unfortunately, ladar systems employing two independent lasers are often prohibitively expensive and bulky.
Furthermore, conventional coherent ladar systems, which often employ Gaussian or Hermite-Gaussian laser beams, cannot accurately measure target rotation. Valuable target data based on the target rotational signature, such as target type, is often unavailable due to the nature of the transmitted laser beams and due to undesirable noise in the laser return signals. Target motion detection with conventional active mode sensor systems is often limited to longitudinal Doppler or target range rate.
Some conventional ladar systems that measure target longitudinal Doppler can infer certain target rotational characteristics when the target rotational axis is not parallel to the laser beam axis. In this case, target rotational motion appears as Doppler phase shifts that are spread about the mean longitudinal Doppler phase shifts associated with the laser returns. The Doppler phase shifts must be analyzed to indirectly infer certain target rotational characteristics. Conventional active mode sensor systems cannot directly measure target rotational rates independent of longitudinal motion. Consequently, such systems are often inapplicable in many important applications, including direct sensing of atmospheric and ocean surface vortices and hard target torsion modes related to target vibration and flexures.
Hence, a need exists in the art for a space-efficient and cost-effective system and method for effectively reducing or eliminating noise, such as platform noise, atmospheric distortion, and background clutter, from return signals in active mode sensing applications. There exists a further need for a system that can effectively measure target rotation to more clearly identify and analyze the target based on target rotational signature.
The need in the art is addressed by the efficient system for measuring target characteristics via a torsion mode beam of electromagnetic energy of the present invention. In the illustrative embodiment, the inventive system is adapted for use with active ladar sensors. The system includes a first mechanism for transmitting a torsion mode beam of electromagnetic energy toward a target. A second mechanism receives a corresponding return beam the electromagnetic energy after reflection from the target and provides a first signal in response thereto. A third mechanism measures rotational characteristics of the target based on the first signal.
In a specific embodiment, the system further includes a fourth mechanism for reducing or eliminating noise in the return beam based on the target rotational characteristics via common mode rejection. A fifth mechanism identifies the type of the target based on the target rotational characteristics via comparison to predetermined target rotational signatures. An additional mechanism selectively alters the mode of the beam of electromagnetic energy between a first mode and a second mode.
The second mechanism includes a detector and a local oscillator. The detector is electrically divided. The local oscillator provides a Gaussian or Laguerre-Gaussian beam for mixing with the first mode and the second mode at the detector to yield corresponding beat frequencies. The first mechanism and second mechanisms employ a transceiver associated with a transmit chain and a receive chain. The transmit chain includes a time-division multiplexer or a spatial multiplexer for selectively transmitting the beam of electromagnetic energy with modes alternating between the first and second modes, or for selectively transmitting first and second spatially separated beams characterized by the first and second modes, respectively. The receive chain includes a corresponding time-division demultiplexer or spatial demultiplexer, respectively, for providing the first signal.
In a more specific embodiment, the second mode is a Gaussian mode. Alternatively, the second mode is a Laguerre-Gaussian torsion mode that is characterized by a mode index different from the first Laguerre-Gaussian torsion mode. Alternatively, the first mode is a left-handed Laguerre-Gaussian torsion mode, and the second mode is a right-handed Laguerre-Gaussian mode.
The third mechanism includes mechanism for determining rotational Doppler fDoppler due to reflection of the beam of electromagnetic energy from the target. The rotational Doppler fDoppler given by the following equation:             f      Doppler        =                  m        ⁢                  xe2x80x83                ⁢        Ω            π        ,
where m is the mode order of the Laguerre-Gaussian beam of electromagnetic energy. xcexa9 is the rotational rate at which the target rotates about a beam axis of the target determination system.
The novel design of the present invention is facilitated by the first and second mechanisms, which implement coherent detection of a torsion mode beam transmitted toward a target and reflected from the target. The torsion content of the torsion mode beam is shifted upon reflection from the target. This shift is used to measure target rotation. By isolating signal components associated with target rotation, common mode noise sources that do not exhibit significant torsion, such as platform vibration and atmospheric distortion are thereby made common mode and, consequently, eliminated. Hence, accurate and direct measurements of target rotation are now possible via the present invention. These direct target rotation measurements facilitate target type identification, tracking, and analysis, and may dramatically improve overall performance of active sensor systems.